The agency said the bullet was "whole or fragmented into smaller pieces."
The FBI said Friday evening a bullet struck former President Donald Trump's ear during his assassination attempt on July 13.
The update to their investigation came after Trump lashed out against FBI Director Christopher Wray for testifying earlier this week that it was still unclear whether Trump was hit with a bullet -- or something else -- at his Pennsylvania rally.
"What struck former President Trump in the ear was a bullet, whether whole or fragmented into smaller pieces, fired from the deceased subject's rifle," the FBI said in its statement.
Trump took to Truth Social Thursday calling out Wray for his testimony a day earlier in Congress where he briefed them on the investigation into the shooting that left one rallygoer dead and two others wounded.
"FBI Director Christopher Wray told Congress yesterday that he wasn't sure if I was hit by shrapnel, glass, or a bullet (the FBI never even checked!) ... " Trump posted in part Thursday night on his Truth Social account.
Trump was reacting to Wray telling House lawmakers on Wednesday that the bureau still hasn't reached a determination as to whether Trump's ear was hit by an actual bullet.
"I think with respect to former President Trump, there's some question about whether or not it's a bullet or shrapnel that hit his ear," Wray said in response to a question from House Judiciary Committee Chairman Jim Jordan asking whether the FBI has accounted for all bullets fired by the shooter.
"It's conceivable -- although as I sit here right now I don't know whether that bullet, in addition to causing the grazing could have also landed somewhere else," Wray said.
As he indicated, the FBI is still examining bullet fragments found at the rally site in Butler, Pennsylvania, according to a source familiar with the investigation, to determine what actually hit the former president.
On Friday, Texas Rep. Ronny Jackson, a former White House physician who served while Trump was president, disputed Wray's testimony in a letter on his congressional letterhead.
"There was no evidence it was anything other than a bullet," Jackson wrote in the letter posted to X by Trump's adviser Jason Miller. "Congress should correct the record as confirmed by both the hospital and myself. Director Wray is wrong and inappropriate to suggest anything else."
Jackson writes that he has been treating Trump in the aftermath of the assassination attempt and concurs with the assessment made by the doctors at the Butler Memorial Hospital the day of the shooting.
The hospital has not released any medical reports about Trump's injury, the cause or any treatment Trump received. Nor is there an official account yet from the state or federal government or the Trump campaign, other than two letters from Jackson, an outspoken supporter.
A statement released by the FBI on Thursday said the attempted assassination resulted in Trump's injury, but the exact manner how is still under investigation.
"The FBI's Shooting Reconstruction Team continues to examine evidence from the scene, including bullet fragments, and the investigation remains ongoing."
In his post on Truth Social, Trump insisted it was a bullet that struck him.
"No, it was, unfortunately, a bullet that hit my ear, and hit it hard," Trump wrote. "There was no glass, there was no shrapnel. The hospital called it a 'bullet wound to the ear,' and that is what it was."
At his campaign rallies, Trump now says he "took a bullet for democracy."
In the same social media post on Thursday, Trump also said Wray "knows nothing" and has promised to fire him if he wins a second term.
ABC News' Lalee Ibssa, Kelsey Walsh and Ivan Pereira contributed to this report.